Big Spender Achievement
Spend $50000 in Vehicle Shops (Campaign only).
Vehicle shops are indicated by a green icon with a car/helicopter/boat in the middle. This is a lot to spend, so save your money. Fully completing one Region should net you over $50,000. This is by only buying a few attachments for your guns, ammo when you need it, looting/skinning when possible and then selling it on and finally completing all types of missions.
It's recommend that you start with the cheapest vehicles and then make your may up to the more costly options. To reach the $50,000 target, you'll most likely have to buy most cars, ATVs, some trucks, and lorries. Then, it's up to you on what to buy next, but know that boats and/or helicopters are the next cheapest. Also, take note that not all vehicles are unlocked at the start and those at the bottom of the lists are the more expensive options.